First download and install AutoUpdate from the AutoUpdate website.

Then create an autoprovides.db for the Red Hat version you want to upgrade to. You can download an autoprovides.db from the autoupdate ftpsite. You can create one (from CD) by supplying the following command for each CD.

autoupd --noupdate --createdb autoprovides-redhat.db \
--addtodb /mnt/cdrom/RedHat/RPMS/*.rpm

Afterwards use the autoprovides.db and an FTP-mirror to download and upgrade the necessary packages. This might take a while and once it has started upgrading you don't want it to be interrupted. So running it inside a screen is advisable. You can do this with the following command:

screen autodld --verbose --norepackage --noaddtodb --database autoprovides-redhat.db \
--url ftp://ftp.easynet.be/redhat/redhat/linux/7.3/en/os/i386/RedHat/RPMS/

This command enables verbosity so you can follow it more closely and disables the rollback-mechanisme which is less important for upgrades (and certainly for older machines).

This command took 17 minutes to remotely upgrade a Red Hat 7.2 to a Red Hat 7.3 while still in production. On a local network this would be even faster. After the upgrade is done, you have to manually upgrade the kernel (if desired) and reboot the system. So a maintenance window is still required ;-)

I've been using the same procedure to upgrade older machines with Red Hat 5.2 to Red Hat 6.2 and then to Red Hat 7.2. (I never tried bigger leaps than that.)

You can get more information about AutoUpdate at the AutoUpdate website